The present invention relates to a process for preparing a high molecular weight polyazole, which can be used especially for production of polymer electrolyte membranes, preferably for production of membrane electrode assemblies for fuel cells.
Polymer electrolyte membranes (PEMs) are already known and are especially used in fuel cells. Frequently, sulfonic acid-modified polymers, especially perfluorinated polymers, are employed. A prominent example thereof is Nafion™ from DuPont de Nemours, Willmington USA. For proton conduction, a relatively high water content in the membrane is required, which is typically 4-20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in conjunction with acidic water and the hydrogen and oxygen reaction gases, limits the operating temperature of the PEM fuel cell stack typically to 80-100° C. Under pressure, the operating temperature can be increased to >120° C. Otherwise, higher operating temperatures cannot be achieved without a loss in performance of the fuel cell.
For system reasons, however, higher operating temperatures than 100° C. in the fuel cell are desirable. The activity of the noble metal-based catalysts present in the membrane electrode unit (MEU) is significantly better at high operating temperatures. More particularly, in the case of use of what are called reformates from hydrocarbons, distinct amounts of carbon monoxide are present in the reformer gas, which typically have to be removed by complex gas treatment or gas purification. At high operating temperatures, the tolerance of the catalysts to the CO impurities rises up to several % by volume of CO.
In addition, heat evolves in the operation of fuel cells. Cooling of these systems to below 80° C. can, however, be very costly and inconvenient. According to the power output, the cooling apparatuses can be made much simpler. This means that, in fuel cell systems which are operated at temperatures above 100° C., the waste heat can be utilized much better, and hence the fuel cell system efficiency can be enhanced by power-heat coupling.
In order to attain these temperatures, membranes with novel conductivity mechanisms are generally used. One approach for this purpose is the use of membranes which exhibit electrical conductivity without the use of water. A first development in this direction is detailed, for example, in WO 96/13872. For instance, WO 96/13872 discloses the use of acid-doped polybenzimidazole membranes which are produced by a casting process.
A new generation of acid-containing polyazole membranes which likewise exhibit electrical conductivity without the use of water is described in WO 02/088219. This application discloses a proton-conducting polymer membrane based on polyazoles, which is obtainable by a process comprising the following steps:    A) mixing one or more aromatic tetraamino compounds with one or more aromatic carboxylic acids or esters thereof which comprise at least two acid groups per carboxylic acid monomer, or mixing one or more aromatic and/or heteroaromatic diaminocarboxylic acids, in polyphosphoric acid to form a solution and/or dispersion    B) applying a layer using the mixture according to step A) on a carrier, optionally on an electrode,    C) heating the flat structure/sheet obtainable according to step B) under inert gas to temperatures of up to 350° C., preferably up to 280° C., to form the polyazole polymer,    D) treating the membrane formed in step C) until it is self-supporting, preferably by partial hydrolysis.
The polyphosphoric acid used in step A) typically has a content, calculated as P2O5 (by acidimetric means), of at least 83%.
To adjust the viscosity, the solution can optionally be admixed with phosphoric acid (conc. phosphoric acid, 85%).
The examples describe numerous syntheses in a polyphosphoric acid having a content, calculated as P2O5 (by acidimetric means), of 83.4%.
The condensation of the monomers is generally initiated by a temperature ramp from 120° C. up to 220° C., the high molecular weight polymers being formed at the high temperatures at the end.
In example 3, a monomer mixture with a monomer solids content of 11.2% is heated stepwise actually to 240° C.
Some of the batches are subsequently diluted with conc. phosphoric acid and then optionally heated to 240° C. Under the conditions specified in this context, however, no further significant polycondensation takes place.
The content of the resulting solutions, calculated as P2O5 (by acidimetric means), is either at most 70.487752% (=theoretical H3PO4 concentration: 97.3%; example 5) or at least 75.465388% (=theoretical H3PO4 concentration: 104.2%, example 3).
The intrinsic viscosity of the polymers at 30° C. is 2.9 dl/g or less.
For the application, however, higher molecular weights (inherent viscosities) would be desirable. These could in principle be attained by heating the reaction mixture to higher temperatures than 220° C. and/or using a polyphosphoric acid with a higher P2O5 content. However, controlling the polycondensation reaction in the batches described in this publication under such conditions presents considerable problems since the polycondensation rate increases markedly with rising temperature or higher P2O5 concentration. The result is that the polycondensation proceeds extremely rapidly and can no longer stopped in a defined manner, for example at a given molecular weight. On the contrary, the significant rise in viscosity of the reaction mixture can lead in the worst case to the solidification of the reaction mixture within the reactor. The resulting compositions then no longer flow and cannot be discharged from the reactor either. In the case of large batch sizes, reactors then have to be dismantled and cleaned in a costly and inconvenient manner. Furthermore, the reactor is temporarily unusable for further reactions, and the mechanical removal of solidified reaction product can lead to damage to coated vessel walls. Simple cleaning by adding a solvent is likewise impossible since the tank is always almost completely filled in the course of the reaction.